How to prevent render cracking: for building designers
To avoid the risk of cracking, it is essential that certain design considerations are taken into account. This section, aimed at the building designer, focuses on good practice in design.
The choice of substrate to receive a render finish is important to maximise bond and help reduce cracking. The main factors to consider when choosing a substrate to be rendered are:
Strength: The substrate, including joints, must be stronger than the render and should be able to adequately support and restrain it.
Suction: Good adhesion of a render is dependent on good porosity of the substrate. If the suction is too high the render will not hydrate sufficiently. If it is too little there will not be a good bond.
Texture: To ensure a good bond the substrate should have an open texture, otherwise weberend aid will need to be used.
Typically, a medium-density (1350–1450kg/m³, 3.6–7.3kN), general purpose, concrete block with a mortar cement:lime:sand ratio of 1:1:6 will satisfy these requirements. If other substrates are considered it is recommended to contact Weber’s technical team for design advice.
The use and placement of movement joints is essential to avoid cracking. Guidance on the placement of movement joints should be obtained from the specified block manufacturer.
This advice should be used in conjunction with BS EN 1996(EC6) Code of practice for design of masonry, BS 6093:2006 + A1:2013.
Code of practice for design of joints and jointing in building construction, and the Concrete Block Association’s guide to movement control. The main areas to include movement joints are as follows, however this will vary depending on the type of block and construction type:
Changes in the wall height or thickness
Changes of loading
Abutments of walls and junctions with dissimilar materials
Zones that are 1 to 3 metres from corners
Up to every 6 metres for unreinforced blockwork
To break up masonry panels so they do not exceed a length:height ratio of 3:1
The adequate use of movement joints will help reduce stress build up in the substrate and thus reduce the risk of cracking. Movement joints must also be followed through the render.
Mortar bed reinforcement
The use of mortar bed reinforcement helps increase the resistance of substrates to cracking and therefore it is essential that it is specified in the building design. There are a large range of masonry reinforcement products designed specifically to control cracking in masonry as covered by BS 5628 Part 3. It is imperative that the type of reinforcement used is structural quality (ladder type).
Mortar bed reinforcement can be used to reduce the frequency of movement joints as well as improving the lateral stability of the wall.
However, mortar bed reinforcement is not a substitute for using movement joints. The examples below show how movement joint frequency can be reduced when using mortar bed reinforcement:
If a movement joint is planned for every 9 metres, mortar bed reinforcement will be needed every other bed joint i.e. 450mm for the entire 9 metre length.
If a movement joint is planned for every 12 metres, mortar bed reinforcement will be needed every bed joint i.e. 225mm for the entire 12 metre length.
Where there are slender panels or panels with large openings, extra mortar bed reinforcement should be considered.